We present a protocol to evaluate the balance between glutamate release and clearance at single corticostriatal glutamatergic synapses in acute slices from adult mice. This protocol uses the fluorescent sensor iGluu for glutamate detection, a sCMOS camera for signal acquisition and a device for focal laser illumination.
Synapses are highly compartmentalized functional units that operate independently on each other. In Huntington's disease (HD) and other neurodegenerative disorders, this independence might be compromised due to insufficient glutamate clearance and the resulting spill-in and spill-out effects. Altered astrocytic coverage of the presynaptic terminals and/or dendritic spines as well as a reduced size of glutamate transporter clusters at glutamate release sites have been implicated in the pathogenesis of diseases resulting in symptoms of dys-/hyperkinesia. However, the mechanisms leading to the dysfunction of glutamatergic synapses in HD are not well understood. Improving and applying synapse imaging we have obtained data shedding new light on the mechanisms impeding the initiation of movements. Here, we describe the principle elements of a relatively inexpensive approach to achieve single synapse resolution by using the new genetically encoded ultrafast glutamate sensor iGluu, wide-field optics, a scientific CMOS (sCMOS) camera, a 473 nm laser and a laser positioning system to evaluate the state of corticostriatal synapses in acute slices from age appropriate healthy or diseased mice. Glutamate transients were constructed from single or multiple pixels to obtain estimates of i) glutamate release based on the maximal elevation of the glutamate concentration [Glu] next to the active zone and ii) glutamate uptake as reflected in the time constant of decay (TauD) of the perisynaptic [Glu]. Differences in the resting bouton size and contrasting patterns of short-term plasticity served as criteria for the identification of corticostriatal terminals as belonging to the intratelencephalic (IT) or the pyramidal tract (PT) pathway. Using these methods, we discovered that in symptomatic HD mice ~40% of PT-type corticostriatal synapses exhibited insufficient glutamate clearance, suggesting that these synapses might be at risk to excitotoxic damage. The results underline the usefulness of TauD as a biomarker of dysfunctional synapses in Huntington mice with a hypokinetic phenotype.
The relative impact of each synaptic terminal belonging to a "unitary connection" (i.e., the connection between 2 nerve cells) is typically assessed by its influence on the initial segment of the postsynaptic neuron1,2. Somatic and/or dendritic recordings from postsynaptic neurons represent the most common and, until now, also the most productive means to clarify information processing under a top-down or vertical perspective3,4,5. However, the presence of astrocytes with their discrete and (in rodents) non-overlapping territories may contribute a horizontal perspective that is based on local mechanisms of signal exchange, integration and synchronization at synaptic sites6,7,8,9,10.
Because it is known that astroglia play, in general, a major role in the pathogenesis of neurodegenerative disease11,12 and, in particular, a role in the maintenance and plasticity of glutamatergic synapses13,14,15,16, it is conceivable that alterations in synaptic performance evolve in accordance with the state of astrocytes in the shared target area of afferent fibers with diverse origin. To further explore the target-/astroglia-derived local regulatory mechanisms in health and disease, it is necessary to evaluate individual synapses. The present approach was worked out to estimate the range of functional glutamate release and clearance indicators and to define criteria that may be used to identify dysfunctional (or recovered) synapses in brain areas most closely related to movement initiation (i.e., first of all in the motor cortex and dorsal striatum).
The striatum lacks intrinsic glutamatergic neurons. Therefore, it is relatively easy to identify glutamatergic afferents of extrastriatal origin. The latter mostly originate in the medial thalamus and in the cerebral cortex (see17,18,19,20 for more). Corticostriatal synapses are formed by the axons of pyramidal neurons localized in cortical layers 2/3 and 5. The respective axons form bilateral intra-telencephalic (IT) connections or ipsilateral connections via a fiber system that more caudally constitutes the pyramidal tract (PT). It has further been suggested that IT- and PT-type terminals differ in their release characteristics and size21,22. In view of these data, one could also expect some differences in the handling of glutamate.
The striatum is the most affected brain area in Huntington's disease (HD)5. Human HD is a severe genetically inherited neurodegenerative disorder. The Q175 mouse model offers an opportunity to investigate the cellular basis of the hypokinetic-rigid form of HD, a state that has much in common with parkinsonism. Starting at an age of about 1 year, homozygote Q175 mice (HOM) exhibit signs of hypokinesia, as revealed by measuring the time spent without movement in an open field23. The present experiments with heterozygote Q175 mice (HET) confirmed the previous motor deficits observed in HOM and, in addition, showed that the observed motor deficits were accompanied by a reduced level of the astrocytic excitatory amino acid transporter 2 protein (EAAT2) in the immediate vicinity of corticostriatal synaptic terminals24. It has therefore been hypothesized that a deficit in astrocytic glutamate uptake could lead to dysfunction or even loss of respective synapses25,26.
Here, we describe a new approach that allows one to evaluate single synapse glutamate clearance relative to the amount of the released neurotransmitter. The new glutamate sensor iGluu was expressed in corticostriatal pyramidal neurons. It was developed by Katalin Török27 and represents a modification of the previously introduced high-affinity but slow glutamate sensor iGluSnFR28. Both sensors are derivatives of the enhanced green fluorescent protein (EGFP). For spectral and kinetic characteristics, see Helassa et al.27. Briefly, iGluu is a low-affinity sensor with rapid de-activation kinetics and therefore particularly well suited to study glutamate clearance at glutamate-releasing synaptic terminals. The dissociation time constant of iGluu was determined in a stopped-flow device, which rendered a Tauoff value of 2.1 ms at 20 °C, but 0.68 ms when extrapolated to a temperature of 34 °C27. Single Schaffer collateral terminals probed at 34 °C with spiral laser scanning in the CA1 region of organotypic hippocampal cultures under a 2-photon microscope exhibited a mean time constant of decay of 2.7 ms.
All work has been carried out in accordance with the EU Directive 2010/63/EU for animal experiments and was registered at the Berlin Office of Health Protection and Technical Safety (G0233/14 and G0218/17).
NOTE: Recordings from Q175 wild-type (WT) and heterozygotes (HETs) can be performed at any age and sex. Here we studied males and females at an age of 51 to 76 weeks.
1. Injection of the Glutamate Sensor iGluu for Expression in Corticostriatal Axons
2. Search for Glutamatergic Terminals Expressing the Glutamate Sensor iGluu
3. Visualization of Glutamate Release and Clearance
Identification of two types of corticostriatal glutamatergic varicosities
IT and PT afferents originate in layer 2/3 and 5, respectively, and exhibit differential ramification and termination patterns in the ipsilateral and contralateral (IT terminals only) striatum. Still little is known about the properties of glutamate release and clearance under repetitive activation conditions as observed during the initiation of movements, but it is well documented that the respective glutamate-releasing varicosities differ in size22. Applying a size criterion, it was found that IT and PT terminals exhibit contrasting forms of short-term plasticity24. At stimulus intervals of 50 ms, the smaller IT terminals were prone to paired pulse depression (PPD) while the larger PT terminals showed paired pulse facilitation. This difference was also observed at shorter intervals (20 ms) and throughout a series of 6 pulses. Figure 3 and Figure 6 illustrate these experiments where synaptic glutamate release was elicited via the action potential mechanism at physiological Ca2+/Mg2+ concentration.
Identification of dysfunctional synapses in mice with advanced Huntington's disease
Neurodegenerative diseases such as Alzheimer's, Parkinson's and Huntington's disease are characterized by an ever progressing loss of glutamatergic synapses36. Novel therapies aim to impede or even reverse this fatal progression. What exactly triggers the disappearance of a synapse, and when, is largely unknown. Further insight can be expected from studies that offer criteria for (a) the vital identification of a particular class of glutamatergic synapses and (b) the detection of dysfunctional versus normally performing contacts. Here it will be shown how the TauD values obtained from PT-type of terminals were used to estimate the fraction of dysfunctional synapses in Q175 heterozygotes with an identified motor phenotype.
Prior to the single synapse imaging experiments, the mice were submitted to a rapid but rather robust test for alterations in their exploratory behaviors. This test is called "step-over test". The animal was placed into the center of a Petri dish (of 185 mm diameter and 28 mm wall height). The test was recorded with a video camera. Using offline analysis, one can determine the time between the take-off of the experimenter's hand and the moment when the animal has all 4 feet out of the dish. Plotting the data from over 100 WT and Q175 HET at ages between 12 and 18 months suggests that mice with a step-over latency of >300 s can be diagnosed as hypokinetic. Figure 7A illustrates a significant positive correlation between the results obtained for the total path run in the open field and the step-over latency.
Single synapse iGluu imaging showed that these symptomatic HD mice exhibited a deficit in the speed of juxtasynaptic glutamate decay as reflected in the TauD values from single (or first in a sequence) stimuli (Figure 7B,C). In WT, such prolongation was only observed after the application of a selective non-transportable inhibitor of glutamate uptake — DL-threo-β-benzyloxyaspartic acid (TBOA, Figure 7D,E). This suggests a role of astrocytic glutamate transporters in the regulation of synaptic glutamate clearance. Changes in the diffusion of Glu in the perisynaptic space have not been found24. But, of course, much more work is needed to actually identify the cause of slowed glutamate clearance in HD as well as in other forms of parkinsonism. Apart from changes in the astrocyte proximity9 and reduced slc1A2 expression37, one may as well consider disease-related instability of EAAT2 in the plasma membrane of the perisynaptic astrocyte processes (PAPs). This might be a result of changes in the EAAT2 interactome. Indeed, recent mass spectroscopy experiments in the lab point to a loss of EAAT2-dystrophin interaction in striatal astrocytes (Hirschberg, Dvorzhak, Kirchner, Mertins and Grantyn, unpublished).
Very little is known regarding the timeline of synaptic dysfunction with progression of HD, but it is very likely that healthy synapses co-exist with already impaired ones. In searching for a classification criterion, we examined the TauD data from different mice. For this purpose, the amplitude and TauD values from 3 consecutive paired trials were normalized to the first response (see Figure 6F for an experimental scheme), and the probability of occurrence of a given TauD value was compared in age-matched WT versus Q175 HET (Figure 6G). It was found that in WT TauD never exceeded 15 ms, while in symptomatic Q175 HET, 40% of the synapses exhibited TauD values between 16 and 58 ms, despite a tendency for a reduction in the amount of released glutamate (Figure 6H,I). TauD might then be regarded as a biomarker for dysfunctional synapses in HD and further be used to verify functional recovery in experiments targeting astrocytic glutamate transport.
Figure 1: Identification of iGluu-positive varicosities. (A) Fluorescence image obtained with a 510 nm high pass filter ("yellow image"). (B) Same view field acquired with a 600 nm high pass filter ("red image"). Note that that the spot marked with black arrowhead has disappeared in (B). Overlay of (A) and (B). White arrow = autofluorescence, black arrow = iGluu-positive varicosity. (D) Image obtained by subtraction of (B) from (A). The autofluorescent spots are dark and the iGluu+ spot is bright. Please click here to view a larger version of this figure.
Figure 2: Movie still from a slow-motion video (slowdown factor 1240x). Upper row: Images from WT (left), Q175 HET (middle) and HOM (right). Lower row: Respective iGluu transients from the pixel with the highest glutamate elevation (Maximal ΔF/F). The red cursor indicates the point on the transient corresponding to the image above the trace. note prolonged elevation of iGluu fluorescence (red pixels and ΔF/F transients). Modified and reprinted with permission from Dvorzhak et al.24 Please click here to view a larger version of this figure.
Figure 3: Extraction of functional indicators from single synapse images of the genetically encoded ultrafast Glu sensor iGluu in corticostriatal neurons. (A, D) Example of a PT (A) and IT (D) bouton with the respective iGluu fluorescence at rest (left) and at the peak of an AP-mediated iGluu response (right). (B, C, E, F) iGluu responses recorded from the bouton shown in (A, D); Experiment in 2 mM Ca2+ and 1 mM Mg2+. (B) Simultaneous recording of the stimulation current (upper trace) and mean intensity of supra-threshold pixels (bottom trace). Same time scale for all traces. Peak amplitudes (between dotted red horizontal lines) and a monoexponential function fitted to the decay from this peak (red overlay). The corresponding TauD (τ) values are shown next to the fitting curves. (E, F) Plot of spread against time. Peak spread: difference between dotted red horizontal lines. Please click here to view a larger version of this figure.
Figure 4: Characteristics of a glutamate-induced iGluu transient (A–E) as opposed to a displacement artifact (F–J). (A, B) and (F, G) show the absolute fluorescence intensity at rest (A, F) and after stimulation (B, G) in arbitrary units (au). (C, H) Fluorescence change in percent of the resting fluorescence prior to stimulation (ΔF/F%). (D, I) Superposition of the iGluu transients from all pixels in arbitrary units. (E, J) Superposition of the iGluu transients from all pixels in ΔF/F %. In the case of synaptic glutamate release, the pixels next to the resting terminal exhibit a fluorescence increase after stimulation, whereas in the case of out-of-focus shifts the brightest pixel at rest merely change their position in the ROI, without an accompanying increase in the over-all fluorescence intensity of the view-field. A displacement artifact can also be recognized by the appearance of negative ΔF/F signals (J). Please click here to view a larger version of this figure.
Figure 5: Correction for unspecific iGluu response. (A–D) Example of paired synaptic response contaminated by an unspecific background response. (E–H) Same after correction. (A, E) Before stimulation. (B, F) During stimulation. (C, G) After stimulation. (D, H) Corresponding superimposed intensity transients (in ΔF/F) from all pixels of the ROI. The timepoint of acquisition of the images is marked by corresponding small letters over the arrowheads. Note that in panel C the background response is very widespread and slowly decaying. Please click here to view a larger version of this figure.
Figure 6: Distinct size and size-related differences in the amplitude paired pulse ratio (PPR) of the iGluu transient. (A) Simplified scheme of the corticostriatal circuitry22, illustrating the concept of preferential projection of pyramidal tract (PT) neurons to indirect pathway striatal projection neurons (iSPNs) and intratelencephalic (IT) neurons to direct pathway SPNs (dSPNs), with size-differences between the IT and PT terminals. (B) Bimodal distribution of bouton diameters as determined by the supra-threshold resting fluorescence before stimulation. Boutons with diameter ≥0.63 µm were defined as "Large" and assumed to be issued by PT axons. For original images and specimen traces from PT- and IT-type of synapses see Figure 3. (C) Significant positive correlation is seen between the PPR of peak amplitudes and the bouton diameters. Each data point represents the average from the first 3 trials of each bouton. *P < 0.05, **P < 0.01, ***P < 0.001. Please click here to view a larger version of this figure.
Figure 7: Identification of dysfunctional synapses in Q175 mice with a hypokinetic phenotype. (A) Results of motor testing on the day of single synapse imaging. Step-over latencies >300 ms were considered as pathological. At the age tested (average 16 months), 17/54 HET exhibited a pronounced phenotype in the step-over test. With regard to hypokinesia, the step-over test seems to be more sensitive than the open field test. Nevertheless, there was a significant correlation between the outcome of the step-over test (time between placement and barrier crossed with all four feet) and the open field test (i.e., total path run in 5 min). Y = -0.01511X + 458,7; P = 0.0044 (simple regression). (B) Average evoked single synapse iGluu transients normalized to same peak amplitude to illustrate HD-related differences in the clearance of synaptically released glutamate. The respective fitting curves highlight the differences in the duration of the glutamate transients. (C) Quantification of the results from wild-type (grey), HET (red) and HOM (magenta). (D, E) Incubation of WT slices in 100 nM of TFB-TBOA simulated the depression of glutamate clearance observed in HOM. (F) Scheme of synapse activation and data organization. (G) Cumulative histograms of #1 TauD values. (H, I) Plots of normalized #2 responses. Data from 31 WT and 30 HET synapses (all of PT type). In the WT sample all TauDmax values were ≤15 ms. In the HET sample, 40% of synapses exhibited TauDmax values exceeding the 15 ms threshold defined by the longest TauDmax in WT. These graphs emphasize the HD-related differences in the ranges of maximal amplitude (i.e., the value from the pixel with the highest iGluu fluorescence increase) and TauDmax (the TauD of the pixel with the highest elevation). TauDmax values exclusively encountered in HET are shown in red, and amplitude values exclusively seen in WT are shown in grey. *P < 0.05, **P < 0.01, ***P < 0.001. The used statistical tests are indicated next to the respective graph. Please click here to view a larger version of this figure.
The experiments concern a question of general interest — synapse independency and its possible loss in the course of neurodegeneration, and we describe a new approach to identify affected synapses in acute brain slices from aged (>1 year) mice. Taking advantage of the improved kinetic characteristics of the recently introduced glutamate sensor iGluu the experiments illuminate the relationship between synaptic glutamate release and uptake in a way that has not been possible before.
The influence of glutamate clearance on the function and maintenance of synapses is not very well elucidated, although the hypothesis that glutamate-induced excitotoxicity can cause neuron loss and synapse pruning is mentioned in almost any pertinent review on epilepsy, stroke and neurodegenerative diseases38,39,40,41. However, the available evidence is more limited than possibly anticipated from the literature. Confusion is further added by the fact that the selected experimental tools might be insufficient in view of the problems associated with low spatial and temporal resolution when interpreting the results obtained with gross stimulation and recording techniques42,43. This can lead to false negatives discouraging further research, which is more than unfortunate in view of neurologic disorders as severe as Huntington's disease. The present approach ensures better signal discrimination and therefore stronger support of the idea that in HD a significant fraction of corticostriatal synapses exhibits signs of impaired glutamate clearance.
The present results disclosed differences of short-term plasticity within the corticostriatal pathway. Although the latter had been in the focus of numerous studies17,44, it has not been anticipated that the afferents originating from the upper cortical layers would exhibit frequency-dependent depression, in contrast to pyramidal tract afferents originating in layer 5. The latter preferentially showed a frequency-dependent potentiation of release and may therefore be at higher risk for glutamate uptake insufficiency.
Experiments on acute brain slices from adult mice are important for elucidating synaptic alterations at an appropriate age of life. The age and functional state of the preparation is particularly relevant if astrocytes were involved in the mechanism of interest. Here, it is essential that the astrocytes are mature enough to exhibit adult levels of glutamate transporter activity and related chloride homeostasis45.
Single synapse assays will pave the road towards a better understanding of HD-related changes of glutamatergic synaptic transmission in the intact brain46. It has been shown that single synapse resolution can also be achieved in the intact brain, provided that the synapses of interest are localized in the superficial layers of the cerebral cortex47.
Finally, the present experimental approach has the appeal that it can be used by many followers, since the required equipment is still on the low-cost side.
In short, fluorescence signals reporting glutamate release and the juxtasynaptic changes in the concentration of glutamate in the intact brain can provide data unachievable with any electrophysiological method. But as with any new method, this approach has its limitations and disadvantages that have in part been addressed in steps 2.2 and 3.3. The low resting fluorescence of the fast and high affinity iGluu sensor requires a bit of exercise/experience to identify suitable varicosities for further testing. With co-expression of a genetically encoded calcium indicator (GECI) such as XCaMP-R48 and at least two coordinated lasers illumination systems, the search of functional axon terminals would become much easier. In any case, it is critical to expose the preparation as little as possible to the exciting light before and during iGluu recording.
The use of a 473 nm laser (instead of the regular whole field elimination) to elicit iGluu fluorescence is a prerequisite for obtaining sufficient emission but will also cause bleaching. Under the given conditions, iGluu was fully bleached after 10 stimulation/acquisition trials (10 stimulus pairs at an inter-stimulus interval of 50 ms and a repetition rate of 1/10 s). The maximal total illumination time for steady-state data acquisition with the presently used 1 photon laser system and the described setting was approximately 2 s. The bleaching of iGluu is exponential, being very strong during the first 20 ms after illumination start and much slower thereafter. It is therefore advisable to avoid signal acquisition during the first 20 ms of each trial and to acquire not more than a total of 10 response pairs. Responses to single stimuli could be recorded with exposure times of 60–80 ms instead of the presently used 180 ms.
Another critical issue is the expression level of iGluu. In the pioneering study of Helassa et al.27, the viral constructs were applied by electroporation to cultured neurons27, which produces higher expression levels of the sensor and presumably also less bleaching especially if a two-photon laser scanning device can be used instead of a one-photon microscope. Dürst et al.49 report routine acquisition of postsynaptic expression of iGluu in CA1 pyramidal neurons in ~100 trials (each exposed for only 80 ms). However, cultured brain tissue is not an option when the experiments aim at clarifying astrocyte-dependent synaptic functions in the aged brain. A CaMKII-Cre-dependent expression of iGluu using a stronger promoter may provide preparations with stronger expression in fewer cells, thereby increasing the resolution of the method and allowing for the acquisition of more trials per synapse.
The authors have nothing to disclose.
This work was supported by CHDI (A-12467), the German Research Foundation (Exc 257/1 and DFG Project-ID 327654276 – SFB 1315) and intramural Research Funds of the Charité. We thank K. Török, St. George's, University of London, and N. Helassa, University of Liverpool, for the iGluu plasmid and many helpful discussions. D. Betances and A. Schönherr provided excellent technical assistance.
Stereo microsope | WPI | PZMIII | Precision Stereo Zoom Binocular Microscope |
Stereotaxic frame | Stoelting | 51500D | Digital Lab New Standard stereotaxic frame |
High speed drill equipment | Stoelting | 514439V | Foredom K1070 cromoter Kit |
Injection system | Stoelting | 53311 | Quintessential Stereotaxic Injector (QSI) |
Hamilton syringe 5 µl | Hamilton | 87930 | 75RN Syr (26s/51/2) |
Laser positioning system | Rapp OptoElectronic | UGA-40 | UGA-40 |
Blue laser for iGluu excitation | Rapp OptoElectronic | DL-473-020-S | 473 nm laser |
Dichroic mirror for 473 nm | Rapp OptoElectronic | ROE TB-355-405-473 | Dichroic |
1P upright microscope | Carl Zeiss | 000000-1066-600 | Axioskop 2 FS Plus |
Objective 63x/1.0 | Carl Zeiss | 421480-9900 | W Plan-Apochromat |
4x objective | Carl Zeiss | 44-00-20 | Achroplan 4x/0,10 |
Dichroic mirror for iGluu | Omega optical | XF2030 | |
Emission filter for iGluu | Omega optical | XF3086 | |
Dichroic mirror | Omega optical | QMAX_DI580LP | |
Emission filter for autofluorescence subtr. | Omega optical | QMAX EM600-650 | |
sCMOS camera | Andor | ZYLA4.2PCL10 | ZYLA 4.2MP Plus |
Acqusition software | Andor | 4.30.30034.0 | Solis |
AD/DA converter | HEKA Elektronik | 895035 | InstruTECH LIH8+8 |
Aquisition software | HEKA Elektronik | 895153 | TIDA5.25 |
Electrode positioning system | Sutter Instrument | MPC-200 | Micromanipulator |
Electrical stimulator | Charite workshops | STIM-26 | |
Slicer | Leica | VT1200 S | Vibrotome |
Brown/Flaming-type puller | Sutter Instr | SU-P1000 | P-1000 |
Glass tubes for injection pipettes | WPI | 1B100F3 | |
Glass tubes forstimulation pipettes | WPI | R100-F3 | |
Tetrodotoxin | Abcam | ab120054 | TTX |
iGluu plasmid | Addgene | 106122 | pCI-syn-iGluu |
Q175 mice | Jackson Lab | 27410 | Z-Q175-KI |